Innovative swirling flow-type microbubble generator for multi-stage DCMD desalination system: Focus on the two-phase flow pattern, bubble size distribution, and its effect on MD performance

Performance Analysis of Air Gap Membrane Distillation Process Enhanced with Air Injection for Water Desalination

Water scarcity is a global issue, and desalination is an alternative to providing fresh water. Renewable energies could be used in thermal desalination to produce freshwater from high saline concentration solutions. In this paper, the experimental performance of an air-injection-Air Gap Membrane Distillation (AGMD) module is presented. The effect of the operation parameters (saline solution temperature, air flow, and salt concentration) on the distilled water rate was evaluated. The air injection enhanced the distilled water rate by 22% at the highest air flow and a solution flow rate of 80 °C, compared to the conventional condition (without air injection) at a salt concentration of 100,000 ppm. Under the same operating conditions, the increase was 17% at a salt concentration of 70,000 ppm. The maximum distilled water rate was 14.10 L/m2·h at 80 °C and an airflow of 1.5 L/min with the highest salt concentration, while it was also 14.10 L/m2·h at the lower salt concentration was 14.10 L/m2·h. The distilled water quality also improved as the air flow increased, since a conductivity reduction of 66% was observed. With the described mathematical model, 94% of the calculated values fell within ±10% of the experimental data for both salt concentration conditions.

Microbubbles in Food Technology

Microbubbles are largely unused in the food industry yet have promising capabilities as environmentally friendly cleaning and supporting agents within products and production lines due to their unique physical behaviors. Their small diameters increase their dispersion throughout liquid materials, promote reactivity because of their high specific surface area, enhance dissolution of gases into the surrounding liquid phase, and promote the generation of reactive chemical species. This article reviews techniques to generate microbubbles, their modes of action to enhance cleaning and disinfection, their contributions to functional and mechanical properties of food materials, and their use in supporting the growth of living organisms in hydroponics or bioreactors. The utility and diverse applications of microbubbles, combined with their low intrinsic ingredient cost, strongly encourage their increased adoption within the food industry in coming years.

Modified Electrospun Membranes Using Different Nanomaterials for Membrane Distillation

Obtaining fresh drinking water is a challenge directly related to the change in agricultural, industrial, and societal demands and pressure. Therefore, the sustainable treatment of saline water to get clean water is a major requirement for human survival. In this review, we have detailed the use of electrospun nanofiber-based membranes (ENMs) for water reclamation improvements with respect to physical and chemical modifications. Although membrane distillation (MD) has been considered a low-cost water reclamation process, especially with the availability of low-grade waste heat sources, significant improvements are still required in terms of preparing efficient membranes with enhanced water flux, anti-fouling, and anti-scaling characteristics. In particular, different types of nanomaterials have been explored as guest molecules for electrospinning with different polymers. Nanomaterials such as metallic organic frameworks (MOFs), zeolites, dioxides, carbon nanotubes (CNTs), etc., have opened unprecedented perspectives for the implementation of the MD process. The integration of nanofillers gives appropriate characteristics to the MD membranes by changing their chemical and physical properties, which significantly enhances energy efficiency without impacting the economic costs. Here, we provide a comprehensive overview of the state-of-the-art status, the opportunities, open challenges, and pitfalls of the emerging field of modified ENMs using different nanomaterials for desalination applications.

Comprehensive experimental and theoretical investigations on the effect of microbubble two-phase flow on the performance of direct-contact membrane distillation

This study provides a comprehensive and systematic overview of the application of gas-liquid two-phase flow with microbubbles in the feed stream to improve heat and mass transfer in direct-contact membrane distillation (DCMD) processes for seawater desalination. A swirl-flow-type microbubble generator (MBG) was installed at the feed-side inlet of the DCMD module to investigate its effect on transmembrane flux. The maximum improvement in the MBG-assisted DCMD permeation flux was found to be approximately 18% at a lower feed temperature (40 °C) and optimal air flow rate (50 cc/min), and an optimal MBG geometry comprising a swirler, a nozzle tip of diameter 2 mm, and a diffuser at an angle of 30°. The results were observed to be related to the number density of microbubbles less than 100 µm in size, which plays an important role in improving heat and mass transfer in two-phase flow. In addition, the simulation results based on conventional heat transfer correlations of bubbly flow underestimated the experimental results. Therefore, this study also aims to propose and verify a new two-phase flow heat transfer correlation. The proposed correlation considers the effects of bubble size distribution to accurately predict the performance of MBG-assisted DCMD processes.

Electrosprayed CNTs on Electrospun PVDF-Co-HFP Membrane for Robust Membrane Distillation

In this investigation, the electrospraying of CNTs on an electrospun PVDF-Co-HFP membrane was carried out to fabricate robust membranes for the membrane distillation (MD) process. A CNT-modified PVDF-Co-HFP membrane was heat pressed and characterized for water contact angle, liquid entry pressure (LEP), pore size distribution, tensile strength, and surface morphology. A higher water contact angle, higher liquid entry pressure (LEP), and higher tensile strength were observed in the electrosprayed CNT-coated PVDF-Co-HFP membrane than in the pristine membrane. The MD process test was conducted at varying feed temperatures using a 3.5 wt. % simulated seawater feed solution. The CNT-modified membrane showed an enhancement in the temperature polarization coefficient (TPC) and water permeation flux up to 16% and 24.6%, respectively. Field-effect scanning electron microscopy (FESEM) images of the PVDF-Co-HFP and CNT-modified membranes were observed before and after the MD process. Energy dispersive spectroscopy (EDS) confirmed the presence of inorganic salt ions deposited on the membrane surface after the DCMD process. Permeate water quality and rejection of inorganic salt ions were quantitatively analyzed using ion chromatography (IC) and inductively coupled plasma-mass spectrometry (ICP-MS). The water permeation flux during the 24-h continuous DCMD operation remained constant with a >99.8% inorganic salt rejection.

Advances in Membrane Distillation Module Configurations

Membrane Distillation (MD) is a membrane-based, temperature-driven water reclamation process. While research emphasis has been largely on membrane design, upscaling of MD has prompted advancements in energy-efficient module design and configurations. Apart from the four conventional configurations, researchers have come up with novel MD membrane module designs and configurations to improve thermal efficiency. While membrane design has been the focus of many studies, development of appropriate system configurations for optimal energy efficiency for each application has received considerable attention, and is a critical aspect in advancing MD configurations. This review assesses advancements in modified and novel MD configurations design with emphasis on the effects of upscaling and pilot scale studies. Improved MD configurations discussed in this review are the material gap MD, conductive gap MD, permeate gap MD, vacuum-enhanced AGMD/DCMD, submerged MD, flashed-feed MD, dead-end MD, and vacuum-enhanced multi-effect MD. All of these modified MD configurations are designed either to reduce the heat loss by mitigating the temperature polarization or to improve the mass transfer and permeate flux. Vacuum-enhanced MD processes and MD process with non-contact feed solution show promise at the lab-scale and must be further investigated. Hollow fiber membrane-based pilot scale modules have not yet been sufficiently explored. In addition, comparison of various configurations is prevented by a lack of standardized testing conditions. We also reflect on recent pilot scale studies, ongoing hurdles in commercialization, and niche applications of the MD process.

Seawater Desalination Based on a Bubbling and Vacuum-Enhanced Direct Contact Membrane Distillation

A Bubbling and Vacuum-enhanced direct contact membrane distillation (BVDCMD) is proposed to improve the water production rate of the direct contact membrane distillation (DCMD-)based seawater desalination process. Its heat and mass transfer mechanism are theoretically analyzed, and a CFD model is established, which is verified by the published data. Four types of the noncondensable gas, “O2,” “air,” “N2,” and “H2,” are adopted as the bubbling gas, and their process enhancements under different pressure of permeate side, temperature, and NaCl concentration of feed side and flow velocities are investigated. The results show that the permeate flux increased remarkably with the decrease in the viscosity of the bubbling gas, and hence, “H2” is the best option for the bubbling gas, with the permeate flux being enhanced by 144.11% and the effective heat consumption being increased by 20.81% on average. The effective water production rate of BVDCMD is predicted to be 42.38% more than that of DCMD, proving its feasibility in the seawater desalination.